AbstractProto-Germanic (PGmc.)aiin stressed syllables shows varied outcomes in Germanic languages (ā, ē, ei), with many of these developments being conditioned by different phonological contexts. This article presents a reconstruction that unifies this variation by assuming that the monophthongisation spread over ‘Germania’ in two waves with different scopes and directions. The first wave expanded from north to south and was primarily limited to the contexts before -hand -r. A second wave, affecting the remaining instances of PGmc.ai, did not reach Old High German and Old West Nordic. The whole process covered the time between 400 and 900. The monophthongisation of PGmc.aidoes not reflect any structural contrast among the Germanic languages, but the results had a differentiating impact on their vowel systems. The presented reconstruction is consistent with the information from runic inscriptions. It supposes a geographical configuration of tribes in a post-Migration setting.